In accordance with the recent increasing storage capacity in information processing, various types of information recording technologies have been developed. In particular, the surface recording density of an HDD using a magnetic recording technology has been increasing at an annual rate of about 100%. Recently, 2.5-inch-diameter perpendicular magnetic recording disks used in HDDs and the like also have been required to have an information recording capacity of larger than 100 GB per disk. In order to satisfy such a requirement, it is necessary to achieve an information recoding density of higher than 150 Gbits per square inch.
In order to achieve a high recording density in a perpendicular magnetic recording disk used in an HDD or the like, it has been necessary to reduce the size of crystalline magnetic particles constituting a magnetic recording layer for recording information signals and simultaneously to decrease the thickness of the layer. However, in a case of a magnetic disk of a conventionally commercialized in-plane magnetic recording system (also called a longitudinal magnetic recording system or a horizontal magnetic recording system), as a result of the progress in the size reduction of crystalline magnetic particles, thermal stability of recorded signals is deteriorated by the superparamagnetic phenomenon. This causes a so-called thermal fluctuation phenomenon in which the recorded signals are erased. Thus, the reduction in size of crystalline magnetic particles has been a factor that inhibits an increase in recording density of the magnetic disk. In order to solve the inhibitory factor problem, recently, a magnetic disk of a perpendicular magnetic recording system (perpendicular magnetic recording disk) has been proposed.
In the perpendicular magnetic recording system, the axis of easy magnetization of a magnetic recording layer is adjusted so as to be orientated in the direction perpendicular to a surface of a substrate, unlike the case of the in-plane magnetic recording system. The perpendicular magnetic recording system can suppress the thermal fluctuation phenomenon compared to the in-plane recording system and is therefore suitable for increasing the recording density.
In addition, in accordance with such an increase in the information recording density, both the linear recording density (BPI: bit per inch) in the circumferential direction and the track recording density (TPI: track per inch) in the radial direction are growing steadily. Furthermore, a technology for increasing the S/N ratio by narrowing the distance (magnetic spacing) between the magnetic layer of a magnetic disk and the recording/reproduction element of a magnetic head has been investigated. It is recently desired that the flying height of a magnetic head be 10 nm or less.
As one of technologies for thus reducing the magnetic spacing, a DFH (dynamic flying height) head has been proposed. In the DFH head, a magnetic head is thermally expanded by inducing heat in a magnetic head element during the operation of the element so as to slightly protrude in the ABS (air bearing surface) direction. By doing so, the distance between the magnetic head and the magnetic disk is controlled so that the magnetic head can constantly and stably fly with a narrow magnetic spacing.
The perpendicular magnetic recording disk has a medium-protecting layer for protecting the surface of a magnetic recording layer from being damaged when the magnetic head crashes with the perpendicular magnetic recording disk. The medium-protecting layer forms a carbon overcoat (COC), i.e., a coating with a high degree of hardness due to a carbon coating. Furthermore, in some medium-protecting layers, both hard diamond-like bonds of carbon and soft graphite bonds of carbon are present (for example, Patent Document 1). In addition, a technology for producing a diamond-like bond protection film by a CVD (chemical vapor deposition) method is disclosed (for example, Patent Document 2). Furthermore, a technology for enhancing durability of a medium-protecting layer is disclosed (for example, Patent Document 3).
Incidentally, in the perpendicular magnetic recording system, a single-pole-type perpendicular head is employed to generate a magnetic field in the direction perpendicular to the magnetic recording layer, as described above. However, since the magnetic flux emerging from a single-pole end is prone to promptly return to a return magnetic pole on the opposite side, a magnetic field with a sufficient intensity cannot be applied to the magnetic recording layer by using only the single-pole-type perpendicular head. Therefore, a soft magnetic layer is provided under the magnetic recording layer of the perpendicular magnetic recording disk and is used as a path for the magnetic flux. This makes it possible to apply a perpendicular direction magnetic field with a high intensity to the magnetic recording layer.
In addition, a technology preventing the occurrence of spike noise is also known (for example, Patent Document 4). In the technology, the soft magnetic layer is separated into two layers with a spacer layer such that the directions of magnetization are parallel to the perpendicular magnetic recording disk surface and are opposite to each other, that is, to form a so-called AFC (antiferromagnetic exchange coupling) structure. This prevents the occurrence of an enormous magnetic domain in a horizontal direction in the soft magnetic layer and the occurrence of the spike noise due to the leaked magnetic flux in the perpendicular direction, which is generated from the magnetic wall of the magnetic domain.
Furthermore, a lubrication layer is disposed on the medium-protecting layer for protecting the medium-protecting layer and the magnetic head from the crash of the magnetic head. The lubrication layer is formed by, for example, applying and sintering perfluoro polyether.    [Patent Document 1] JP-A-H10-11734    [Patent Document 2] JP-A-2006-114182    [Patent Document 3] JP-A-2005-149553    [Patent Document 4] JP-A-2002-358618